This invention relates to a tomographic device using nuclear magnetic resonance (NMR) phenomena and in particular to techniques for imaging the speed of blood flow in a body.
The principle of the imaging of blood flow consists in applying a gradient magnetic field which has no influences on still standing substances but influences only moving substances, in the direction of the flow and adding various phase information thereto, depending on the speed thereof in order to measure it. A gradient magnetic field G is applied thereto at a point of time .tau..sub.1 in the direction of the blood flow and the inverted gradient magnetic field (-G) is applied thereafter at a point of time .tau..sub.2. The inverted gradient magnetic field means a magnetic field, whose magnitude is not changed and only whose sign is inverted.
Since still standing substances don't move, they feel magnetic fields, whose magnitude is not changed and whose sign is inverted, at points of time .tau..sub.1 and .tau..sub.2. Therefore their influences are cancelled by each other and the substances return to their initial state. On the other hand, since the blood moves, it feels different magnetic fields at the points of time .tau..sub.1 and .tau..sub.2 and their influences are not cancelled, which gives rise to variations in the phase of the spin.
Usually the perpendicular direction to the slice is called the z-direction, the direction of the gradient magnetic field for reading-out, which is the horizontal direction of the image, the x-direction, and the phase-encoded direction, which is the vertical direction of the image, the y-direction. Hereinbelow the principle of the method for imaging three-dimensional moving liquid will be explained, referring to these x, y and z directions.
The combination of the two gradient magnetic fields described above is called a flow encoded pulse, which is used necessarily for the blood measurement using the phase angle.
A method for measuring the blood flowing in the x, y or z direction by using the phase angle is reported e.g. "Verification and Evaluation of Internal Flow and Motion" by R. Moran et al., Radiology, Volume 154, Number 2, pp. 433-441, Feb. 1985.
According to the prior art techniques stated above, the direction of the blood flow, which can be measured exactly by one imaging is restricted to either one of the x, y and z directions.
However, since the direction of the blood flow is generally not known, the components thereof should be measured in the three directions, i.e. x-, y- and z-directions and thus the imaging must be repeated necessarily three times.
Since one imaging by means of a nuclear magnetic resonance imaging device takes 2 to 20 minutes, it is desirable to reduce the number of imagings from the view points of the problem provoked by positional displacements due to movements of the patient during one imaging and the throughput.